> With "moving" I really meant "copying" or "cloning" with the
> additional possibility of deleting the original.  So there should be
> only one question: Can we simply make a copy of a tab using its window
> state (window configurations can't be used because we cannot clone
> windows).  If not, what is the problem?

Implemented in the patch below that deletes the window-configuration
from the copied tab, but leaves its window-state.

>> An additional question to consider is how to interactively select
>> an another frame: by frame name, or using other-frame with a numeric
>> prefix argument?
>
> How is this question related to tabs?

Easier to implement than to explain :)  The same patch uses its ARG
the same way as it's used in 'other-frame' to find ARGth frame
where to copy the tab.

>> (push '("test1" .
>>           (display-buffer-reuse-window
>>            (reusable-frames  . visible)
>>            (use-tabs . t)))
>>         display-buffer-alist)
>
> There is one thing I apparently do not understand yet: When you enable
> 'tab-bar-mode' it is global

Do you think we should have 'global-tab-bar-mode' for all frames,
and 'tab-bar-mode' to enable/disable the tab-bar in every frame
separately?

> - that is any window ever shown on any frame is also in at least one
> of that frame's tabs.  Is that right?  So what would 'use-tabs' mean
> here when every window is in a tab already?

These windows in tabs are in window-configurations and window-states.
Now I installed tab-bar-get-buffer-tab that can be used
in display-buffer-reuse-window to search the buffer
in window-states of tabs when use-tabs is non-nil.

>>>    By default, a new tab starts with the current buffer that was current
>>>    before calling the command that adds a new tab.
>>>
>>> That's confusing, at least.
>>
>> Maybe this is better?
>>
>>    By default, a new tab starts with the buffer that was current
>>    before calling the command that adds a new tab.
>
> The current buffer is IMHO a much too obscure object to consider here.
> Don't you mean the buffer of the selected window?

The manual refers to the default value of tab-bar-new-tab-choice
with its docstring:

  If t, start a new tab with the current buffer, i.e. the buffer
  that was current before calling the command that adds a new tab
  (this is the same what `make-frame' does by default).